1. Technical Field
The present invention relates to a process for preparing a modified zeolite Y having increased stability and preferably also a small zeolite unit cell size.
2. Description of Background Art
In the application of zeolites, e.g. for catalysts for the processing of petroleum, especially for the production of so-called middestillates, it is known that an increased stability and, in particular, a small unit cell size dimension (a.sub.o) improves the performance of the catalyst (compare for example U.S. Pat. No. 4,419,271). In view hereof, it has been endeavoured in different ways to increase the stability and, in particular, to reduce the unit cell size from the one obtained upon synthesizing (usually a.sub.o =24.63-24.70 .ANG.) to as low a value as possible. Thus, GB-A-2,085,861 discloses how zeolite NaY can be modified by a combination of ammonium and aluminum ion-exchange steps It appears that this treatment is capable of increasing the catalyst activity and reducing the unit cell size to 24.45-24.52 .ANG.. However, it was found that this production process gives a low residual ion-exchange degree in respect of the aluminum ions.
GB-A-2,014,970 discloses an ultrahydrophobic zeolite Y which has been given a unit cell size dimension a.sub.o of 24.20-24.45 .ANG. by two ammonium ion-exchange steps with an intermediate calcination step at 550.degree.-800.degree. C. in steam (water vapor).
EP-B-0,028,938 discloses the use of such a modified zeolite for selective conversion of hydrocarbons boiling above 371.degree. C. into midbarrel fuel products having a distillation range of 149.degree.-371.degree. C.
GB-A-2,114,594 describes the selective production of hydrocarbon middistillates by using, e.g. modified zeolite Y which has been calcined in steam at high temperature and has a unit cell size in the range 24.36-24.58 .ANG., especially below 24.40 .ANG..